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This paper attempts to resolve the long-standing contradiction between the observed longevity of oceanic vortices and their theoretical instability. Using the model of quasigeostrophic, two-layer ocean, we show that a vortex in th...
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This paper attempts to resolve the long-standing contradiction between the observed longevity of oceanic vortices and their theoretical instability. Using the model of quasigeostrophic, two-layer ocean, we show that a vortex in the upper layer can be stabilised by a circulation in the lower layer, such that the potential vorticity (PV) there is uniform. It is also argued that the assumption of uniform PV in the 'passive' layer corresponds to the fact that most oceanic vortices are shed by frontal currents and are alien to the well-mixed water masses around them.
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Anticyclonic vortices focus and trap near-inertial waves so that near-inertial energy levels are elevated within the vortex core. Some aspects of this process, including the nonlinear modification of the vortex by the wave, are ex...
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Anticyclonic vortices focus and trap near-inertial waves so that near-inertial energy levels are elevated within the vortex core. Some aspects of this process, including the nonlinear modification of the vortex by the wave, are explained by the existence of trapped near-inertial eigenmodes. These vortex eigenmodes are easily excited by an initial wave with horizontal scale much larger than that of the vortex radius. We study this process using a wave-averaged model of near-inertial dynamics and compare its theoretical predictions with numerical solutions of the three-dimensional Boussinesq equations. In the linear approximation, the model predicts the eigenmode frequencies and spatial structures, and a near-inertial wave energy signature that is characterized by an approximately time-periodic, azimuthally invariant pattern. The wave-averaged model represents the nonlinear feedback of the waves on the vortex via a wave-induced contribution to the potential vorticity that is proportional to the Laplacian of the kinetic energy density of the waves. When this is taken into account, the modal frequency is predicted to increase linearly with the energy of the initial excitation. Both linear and nonlinear predictions agree convincingly with the Boussinesq results.
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Downstream of Drake Passage, the Antarctic Circumpolar Current (ACC) veers abruptly northward along the continental slope of South America. This spins down the ACC, akin to the western boundary currents of ocean gyres. During this...
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Downstream of Drake Passage, the Antarctic Circumpolar Current (ACC) veers abruptly northward along the continental slope of South America. This spins down the ACC, akin to the western boundary currents of ocean gyres. During this northward excursion, the mean potential vorticity (PV) increases dramatically (decreases in magnitude) by up to a factor of 2 along mean geostrophic streamlines on middepth buoyancy surfaces. This increase is driven by drag near the continental slope, or by breaking eddies further offshore, and is balanced by a remarkably steady, eddy-driven decrease of mean PV along these northern circumpolar streamlines in the open ocean. We show how two related eddy processes that are fundamental to ACC dynamics-poleward buoyancy fluxes and downward fluxes of eastward momentum-are also concomitant with materially forcing PV to increase on the northern flank of a jet at middepth, and decrease on the southern flank. For eddies to drive the required mean PV decrease along northern streamlines, the ACC merges with the subtropical gyres to the north, so these streamlines inhabit the southern flanks of the combined ACC-gyre jets. We support these ideas by analyzing the time-mean PV and its budget along time-mean geostrophic streamlines in the Southern Ocean State Estimate. Our averaging formalism is Eulerian, to match the model's numerics. The thickness-weighted average is preferable, but its PV budget cannot be balanced using Eulerian 5-day averaged diagnostics, primarily because the z-level buoyancy and continuity equations' delicate balances are destroyed upon transformation into the buoyancy-coordinate thickness equation.
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Direct observations of the velocity field of a submesoscale coherent vortex in the Greenland Sea by a vessel mounted ADCP, together with a detailed and complete investigation of its hydrographic structure, show that within the edd...
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Direct observations of the velocity field of a submesoscale coherent vortex in the Greenland Sea by a vessel mounted ADCP, together with a detailed and complete investigation of its hydrographic structure, show that within the eddy the hydrographic structure differs from the background and indicate that the entire water column of 3.7 km depth is included in an anticyclonic rotation with maximum azimuthal speeds at roughly 2000 m depth. Within a radius of about 9 km the water layers rotate in a solid body fashion. The lifetime of the eddy is long enough and the exchange between its interior and the background is small enough to result in a persistent and pronounced anomaly with respect to physical as well as biological parameters. Due to the unique possibility in the high latitudes to 'recharge' the eddy by surface forcing during winter and a corresponding long lifetime such eddies represent key areas for future bottom water production. [References: 13]
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The Arabian Sea, influenced by the Indian monsoon, has many unique features, including its basin-scale seasonally reversing surface circulation and the Great Whirl, a seasonal anticyclonic system appearing during the southwest mon...
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The Arabian Sea, influenced by the Indian monsoon, has many unique features, including its basin-scale seasonally reversing surface circulation and the Great Whirl, a seasonal anticyclonic system appearing during the southwest monsoon close to the western boundary. To establish a comprehensive dynamical picture of the Arabian Sea, we utilize numerical model output and design a full vorticity budget that includes a fully decomposed nonlinear term. The ocean general circulation model has 0.1 degrees resolution and is mesoscale eddy-resolving in the region. In the western boundary current system, we highlight the role of nonlinear eddies in the life cycle of the Great Whirl. The nonlinear eddy term is of leading-order importance in this feature's vorticity balance. Specifically, it contributes to the Great Whirl's persistence in boreal fall after the weakening of the southwesterly winds. In the open ocean, Sverdrup dynamics and annual Rossby waves are found to dominate the vorticity balance; the latter is considered as a key factor in the formation of the Great Whirl and the seasonal reversal of the western boundary current. In addition, we discuss different forms of vertically integrated vorticity equations in the model and argue that the bottom pressure torque term can be interpreted analogously as friction in the western boundary and vortex stretching in the open ocean.
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Concentrated poleward flows along eastern boundaries between 2- and 4-km depth in the southeast Pacific, Atlantic, and Indian Oceans have been observed, and appear in data assimilation products and regional model simulations at su...
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Concentrated poleward flows along eastern boundaries between 2- and 4-km depth in the southeast Pacific, Atlantic, and Indian Oceans have been observed, and appear in data assimilation products and regional model simulations at sufficiently high horizontal resolution, but their dynamics are still not well understood. We study the local dynamics of these deep eastern boundary currents (DEBCs) using idealized GCM simulations, and we use a conceptual vorticity model for the DEBCs to gain additional insights into the dynamics. Over most of the zonal width of the DEBCs, the vorticity balance is between meridional advection of planetary vorticity and vortex stretching, which is an interior-like vorticity balance. Over a thinner layer very close to the eastern boundary, a balance between vorticity tendencies due to friction and stretching that rapidly decay away from the boundary is found. Over the part of the DEBC that is governed by an interior-like vorticity balance, vertical stretching is driven by both the topography and temperature diffusion, while in the thinner boundary layer, it is driven instead by parameterized horizontal temperature mixing. The topographic driving acts via a cross-isobath flow that leads to stretching and thus to vorticity forcing for the concentrated DEBCs.
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New analytical techniques for studying the motion of a point vortex in fluid domains bounded by straight walls having an arbitrary number of gaps are presented. By exploiting explicit formulae for the Kirchhoff-Routh path function...
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New analytical techniques for studying the motion of a point vortex in fluid domains bounded by straight walls having an arbitrary number of gaps are presented. By exploiting explicit formulae for the Kirchhoff-Routh path function in multiply connected circular domains, combined with a novel construction of conformal mappings from such circular domains to multiply connected slit domains, the governing Hamiltonians for the motion of a point vortex in a number of physically interesting fluid regions involving walls with gaps are derived. The vortex trajectories in several illustrative cases are computed. These examples include finding the vortex paths around a chain of islands sitting off an infinite coastline, around islands in an unbounded ocean and around a sequence of islands Situated between two headlands.
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This paper examines the stability of vortices in a two-layer ocean on the f-plane. The mean depth h_1 of the upper layer is assumed to be much smaller than the depth h_2 of the lower layer. Using the primitive equations, we derive...
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This paper examines the stability of vortices in a two-layer ocean on the f-plane. The mean depth h_1 of the upper layer is assumed to be much smaller than the depth h_2 of the lower layer. Using the primitive equations, we derive an asymptotic criterion for baroclinic instability of compensated (i.e. confined to the upper layer) vortices. Surprisingly, it coincides exactly with a similar criterion derived from the quasigeostrophic equations [Benilov, E.S., 2003. Instability of quasigeostrophic vortices in a two-layer ocean with thin upper layer. J. Fluid Mech. 475, 303-331]. Thus, to leading order in h_1/h_2, ageostrophy does not affect the stability properties of thin compensated vortices. As a result, whether a vortex is stable or not, depends on its shape, not amplitude (although the growth rate of an unstable vortex does depend on its amplitude).
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Oceanic vortices play an important role in the redistribution of heat, salt and momentum in the oceans. Among these vortices, floating lenses or rings are often met in the meanders of warm currents. In order to better describe the...
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Oceanic vortices play an important role in the redistribution of heat, salt and momentum in the oceans. Among these vortices, floating lenses or rings are often met in the meanders of warm currents. In order to better describe these vortices, we propose here a laboratory study of floating anticyclonic lenses. A small volume of fresh water is gently injected near the surface of a rotating layer of homogeneous salted water. Because of the opposite effects of rotation that tends to generate columnar structures and density stratification that spreads light water on the surface, the vortices take after a rapid transient, a quasi-stationary lenticular, finite-sized three dimensional typical shape given by the hydrostatic and geostrophic balances. Visualization and measurements of this equilibrium shape, aspect ratios and vorticity fields are performed. These measurements permit to compare our laboratory anticyclones to analytical predictions that use first a simple solid body rotation model and then a more realistic isolated Gaussian vorticity field. Finally, a comparison of our models with oceanographic lenses described in the literature is discussed. (C) 2016 Elsevier Masson SAS. All rights reserved.
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The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic meridional overturning circulation, yet a description of the full velocity field across the strait remains incomplete. Here we a...
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The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic meridional overturning circulation, yet a description of the full velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic-velocity sections occupied along the Latrabjarg transect at the Denmark Strait sill, obtained over the time period 1993-2018. The sections provide the first complete view of the kinematic components at the sill: the shelfbreak East Greenland Current (EGC), the combined flow of the separated EGC, and the North Icelandic Jet (NIJ), and the northward-flowing North Icelandic Irminger Current (NIIC). The total mean transport of overflow water is 3.54 ± 0.29 Sv (1 Sv = 10~6m~3s~(-1)), comparable to previous estimates. The dense overflow is partitioned in terms of water mass constituents and flow components. The mean transports of the two types of overflow water-Atlantic-origin Overflow Water and Arctic-origin Overflow Water-are comparable in Denmark Strait, while the merged NlJ-separated EGC transports 55% more water than the shelfbreak EGC. A significant degree of water mass exchange takes place between the branches as they converge in Denmark Strait. There are two dominant time-varying configurations of the flow that are characterized as a cyclonic state and a noncyclonic state. These appear to be wind-driven. A potential vorticity analysis indicates that the flow through Denmark Strait is subject to symmetric instability. This occurs at the top of the overflow layer, implying that the mixing/entrainment process that modifies the overflow water begins at the sill.
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